Bolometers are energy detectors based upon a change in the resistance of materials (called bolometer elements) that are exposed to a radiation flux. The bolometer elements have been made from both metals and semiconductors. In case of the metals, the resistance change is essentially due to a variation in the carrier mobility, which typically decreases with temperature. In contrast, greater sensitivity can be obtained in high-resistivity semiconductor bolometer elements wherein the free-carrier density is an exponential function of temperature; however, thin film fabrication of semiconductor elements for the construction of bolometers is a difficult task.
In FIGS. 1 and 2, there are shown a perspective view and a cross sectional view illustrating a bolometer 100, the bolometer 100 including an active matrix level 110, a support level 120, at least a pair of posts 170 and an absorption level 130.
The active matrix level 110 has a substrate 112 including an integrated circuit (not shown), a pair of connecting terminals 114 and a protective layer 116. Each of the connecting terminals 114 made of a metal is located on top of the substrate 112. The protective layer 116 made of, e.g., silicon nitride (SiN.sub.x), covers the substrate 112. The pair of connecting terminals 114 are electrically connected to the integrated circuit.
The support level 120 includes a pair of bridges 140 made of insulating material, e.g., silicon oxide, each of the bridges 140 having a conduction line 165 formed on top thereof. One end of the conduction line 165 is electrically connected to the respective connecting terminal 114 through a via hole 155.
The absorption level 130 is provided with a bolometer element 185 made of titanium (Ti), an absorber 195 made of insulating material, e.g., silicon oxide (SiO.sub.2) or silicon oxy-nitride (SiO.sub.x N.sub.y) and an IR absorber coating 197 formed on top of the absorber 195. The bolometer element 185 has a serpentine shape for increasing its resistivity.
Each of the posts 170 is placed between the absorption level 130 and the support level 120. Each of the posts 170 includes an electrical conduit 172 made of a metal, e.g., titanium (Ti), and surrounded by an insulating material 174 made of, e.g., silicon oxide (SiO.sub.2) or silicon oxy-nitride (SiO.sub.x N.sub.y). Top end of the electrical conduit 172 is electrically connected to one end of the serpentine bolometer element 185 and bottom end of the electrical conduit 172 is electrically connected to the conduction line 165 on the bridge 140, in such a way that both ends of the serpentine bolometer element 185 in the absorption level 130 is electrically connected to the integrated circuit of the active matrix level 110 through the electrical conduits 172, the conduction lines 165 and the connecting terminals 114. When exposed to infra-red radiation, the resistivity of the serpentine bolometer element 185 changes, causing a current and a voltage to vary, accordingly. The varied current or voltage is amplified by the integrated circuit, in such a way that the amplified current or voltage is read out by a detective circuit (not shown).
There are certain shortcomings associated with the above described the infra-red bolometer 100. For example, since the absorption level 130 is structurally asymmetric, that is, the length of bolometer element 185 formed in row direction is different from that of bolometer element 185 formed in column direction, compression stress built up inside the absorber 195 gets unevenly distributed, bending the absorber 195 in one direction, as shown in FIG. 3, which will, in turn, reduce the overall absorption efficiency of the infra-red bolometer 100 decreases.